Electrical stimulation may be used for pain management. One such therapy is transcutaneous electrical nerve stimulation (TENS) therapy, which provides short-term pain relief. Electrical nerve stimulation and electrothermal therapy may also be used to relieve pain associated with various conditions, including back pain. Additionally, intradiscal electrothermal therapy (IDET) is a treatment option for patients with low back pain resulting from intervertebral disc problems.
Pain is typically attributable to a stimulus on nerve endings, which transmits signal impulses to the brain. This type of pain is referred to as nociceptive pain, a somatic sensation of pain, where a patient is made aware of potential tissue damage by neural processes encoding and processing noxious stimuli. The sensation is initiated by nociceptors that detect mechanical, thermal, or chemical changes above a pain threshold. Once stimulated, a nociceptor transmits a signal within the central nervous system through neurons. Each neuron transmits impulse information about the stimulus on the nerve endings along portions of the central nervous system transmission pathway.
Non-nociceptive pain is referred to as neuropathic pain or neuralgia. Neuralgia is pain produced by a change in neurological structure or function. Unlike nociceptive pain, neuralgia exists with no continuous nociceptive input. That is, neuralgia may develop without any actual impending tissue damage. Neuralgia may involve a disease of the nervous system, including an underlying disease process or injury, or from inflammation, infection, and compression or physical irritation of a nerve. Neuralgia is a form of chronic pain and can be extremely difficult to diagnose and treat.
Pain sensations may be gated naturally, such as when pain sensation is inhibited by activation of large diameter afferent neurons activated by vibration, such as when someone burns their hand, and it is involuntarily shaken in response. Transcutaneous electrical nerve stimulation also employs this technique by applying electrical nerve stimulating impulses from an external stimulator to reduce transmission of pain signals to the brain.
Transcutaneous electrical nerve stimulation (TENS) therapy may be used to treat both nociceptor pain and neuralgia. In TENS therapy, an electrical current is applied through the skin near the source of pain. The current is often delivered via electrodes. The current from the electrodes stimulates nerves in the affected area and sends signals to the brain that activate receptors in the central nervous system to reduce normal pain perception.
In a “Textbook of Pain” (Butler & Tanner Ltd., 3rd Ed. 1994, pp. 59-62), authors Melzack and Walls proposed a gate theory to describe the manner in which transcutaneous electrical nerve stimulation devices interfere with pain. Melzack and Walls suggest that TENS devices generate an artificial abnormal noise on the neural pathways that are shared with the pain fibers conducting the real pain impulses. When the transmission of pain impulses from that region of the body are received by the central nervous system, the impulses are “gated.” That is, the transmission of the pain impulses is altered, changed, or modulated in the central nervous system by the artificial signals. As the central nervous system receives the barrage of signals from the stimulated region of the body, a neurological circuit closes a gate and stops relaying the pain impulses to the brain.
Gating is affected by the degree of activity in the large diameter and the small diameter nerve fibers. Nerve transmissions carried by large nerve fibers travel more quickly than nerve transmissions carried by small nerve fibers. As such, transcutaneous electrical nerve stimulation to large nerve fibers travel to the brain more quickly and are more powerful than pain impulses carried by smaller nerve fibers. Thus, the transcutaneous electrical impulses often arrive at the brain sooner than the pain nerve impulses, and the sensation of the large nerves overrides and blocks out the sensations from the smaller pain nerves. That is, impulses along the larger fibers tend to block pain transmission (close the gates) and more activity in the smaller fibers tends to facilitate transmission (open the gates). The gating mechanism in the spinal cord is affected by descending impulses from the brain. Large fibers may activate specific cognitive processes in the brain, which then influence the gate by downward (descending) impulse transmission.
Another theory regarding the pain reducing effect of transcutaneous electrical nerve stimulation devices is based on the understanding of serotonin and other chemical neurotransmitters that participate in the pain and the pain reduction processes in the central nervous system. Transcutaneous electrical nerve stimulation devices produce their effects by activating opioid receptors in the central nervous system. For example, high frequency transcutaneous electrical nerve stimulation activates delta-opioid receptors both in the spinal cord and supraspinally in the medulla, while low frequency transcutaneous electrical nerve stimulation activates mu-opioid receptors both in the spinal cord and supraspinally. Further high frequency transcutaneous electrical nerve stimulation reduces excitation of central neurons that transmit nociceptive information, reduces release of excitatory neurotransmitters such as glutamate, and increases the release of inhibitory neurotransmitters, including GABA, in the spinal cord, and activates muscarinic receptors centrally to produce analgesia. Low frequency TENS also releases serotonin and activates serotonin receptors in the spinal cord, releases GABA, and activates muscarinic receptors to reduce excitability of nociceptive neurons in the spinal cord.
By applying an electrical field to nervous system tissue, electrical stimulation can effectively reduce or mask certain types of pain transmitted from regions of the body. Pain perception may be inhibited by the applied electrical signals interfering with nerve transmission pathways carrying a pain transmission.
However, electrical stimulation intended to manage or control a pain condition may inadvertently interfere with other nerve transmission pathways in adjacent nervous tissue. Because neurostimulation devices must apply electrical energy across a wide variety of tissues and fluids, the amount of stimulation energy needed to provide the desired amount of pain relief is difficult to precisely control. As such, increasing amounts of energy may be required to ensure sufficient stimulation energy reaches the desired stimulation area. However, as the applied stimulation energy increases, so does the likelihood of damage of surrounding tissue, structures, or neural pathways.
In order to provide pain relief, the targeted tissue must be stimulated, but the applied electrical energy should be properly controlled, and the amount and duration of energy applied to surrounding or otherwise non-targeted tissue must be minimized or eliminated. An improperly controlled electric pulse may not only be ineffective in controlling or managing pain, but it may inadvertently interfere with the proper neural pathways of adjacent spinal nervous tissue.